The invention concerns a process for construction of cut-off walls, as well as geotextile mats.
Vertical cut-off walls produced in the ground are used as watertight soil enclosures, as cutoff walls in retaining dams and as bank protection in water bodies.
Since the late 1970s vertical cut-off walls have also been used as casings for originally unsealed waste disposal sites. The essential function of a cut-off wall is to interrupt ground water flow. In this fashion the discharge of pollutants from waste disposal sites is interrupted and seepage is contained within said sites.
Cut-off walls are introduced, for example, in a trench wall process (one-phase process or two-phase process). In the two-phase process, excavation initially occurs using a bentonite/water suspension as liquid. After the final depth for the cut-off wall is reached, the bentonite suspension is replaced by the cut-off wall composition, which forms the cut-off wall after hydraulic setting.
In the one-phase process the support liquid is simultaneously the cut-off wall composition. A suspension of water-swellable bentonite (about 30-60 kg/m.sup.3), cement (about 150-300 kg/m.sup.3) and water (about 650-950 kg/m.sup.3) is used as the cut-off wall composition. Excavation of the cut-off wall occurs with this suspension. After the final depth is reached, the supporting liquid/cut-off wall composition remains in the trench and sets hydraulically (David Urban, Vertical Sealing of Landfills and Contaminated Soils, BBR 3/93, Vol. 44, p. 102 ff).
By design the cut-off walls so produced exhibit very limited water permeability. The so-called combination cut-off walls were developed to reduce water permeability and also permeation of pollutants through the cut-off wall. In this case HDPE sheets (high-density polyethylene) are introduced vertically into the cut-off wall composition before it hydraulically sets. Joining of the individual webs occurs via locking mechanisms that are optionally sealed with additional sealing composition. In this fashion the cut-off wall is sealed over the entire surface by a water-impermeable HDPE sheet.
DE-C-3 605 252 describes a trench wall for vertical sealing of liquid-conducting and/or gas-conducting layers with a trench cut into the ground down to an impermeable horizontal layer. The trench is filled with a composition, like bentonite, and with a seal immersed in that composition composed of several water-impermeable sealing webs that can be joined at the contact points by a locking mechanism consisting of two hook-shaped interlocking parts.
DE-A-3 504 119 describes trench walls with sealing webs that are joined via locking elements (tubes fit into each other).
EP-B-0 278 343 describes a device for joining sealing sheets which is incorporated into a cut-off wall. The sealing sheets consist of liquid-impermeable plastic tightly joined via a special locking mechanism.
Ensuring mechanical integrity of the film after incorporation is problematical in these references and the long-term stability of the plastic sealing web is also questionable.
A requirement therefore exists for an additional sealing element in which mechanical damage that can occur during incorporation of the webs can be limited. In addition, this type of sealing element must be stable for longer periods of time than conventional sealing elements.
DE-A-4 221 329 describes bentonite geotextile mats that contain outlet slits for water-swollen bentonite in their edge regions where the mats are superimposed during production. The emerging bentonite produces tight seals between the mats in the regions of overlap. However, the sealing mats are not used to produce cut-off walls, but for horizontal sealing of landfills. In addition, they contain no locking mechanism to join one web to another web.
U.S. Pat. No. 5 173 344 describes the tight joining of bentonite geotextile mats. These are laid out to overlap, such that a cylindrical strand, consisting of bentonite enclosed by a polyester fabric, is introduced in the region of overlap. On entry of water the bentonite present in the strip swells, causing the enclosure to burst. The bentonite is distributed in the region of overlap, during which the sealing webs are supposed to be joined tightly together. Tight joining, however, is not guaranteed if the mats are exposed to tensile forces. In particular, these mats are not suitable for introduction vertically as tensile forces act between the individual webs during insertion.
U.S. Pat. No. 4 565 468 discloses a bentonite geotextile mat for horizontal application in which the individual webs are joined by a type of adhesive film. The sealing effect, however, is not guaranteed as the adhesive film does not possess adequate strength under mechanical stress, which can occur during introduction of the mats into the vertical cut-off walls.
U.S. Pat. No. 4 048 373 describes a sealing panel that contains a bentonite layer between two paper board layers in which salts of phosphoric acid, boric acid and acetic acid, supplemented by water-soluble polymers, are contained. This panel is not a bentonite geotextile mat used for incorporation vertically in cut-off walls. Further, the individual panels cannot be rigidly joined.
A bentonite panel is disclosed by U.S. Pat. No. 4 103 499 in which salts of acetic acid, phosphoric acid or boric acid, and optionally hydrophilic polymers, are contained in the bentonite layer. Addition of these salts is designed to improve tightness for salt-containing liquids. However, the panel cannot be used for incorporation in trench walls because no locking mechanisms are provided on the side edges.
A composite mat, especially for rock, dam and landfill construction is disclosed by DE-U-9 400 650. It essentially consists of strips of synthetic or natural fibers forming a lattice mat woven together in the fashion of warp and weft threads and at least one coating that can be joined to the lattice mat. The lattice mats have reinforcements in some places and along the edge region of the lattice mat. The coating is designed as a joining element. However, the composite mat serves only to support regions threatened by erosion and has no sealing effect.
The objective of the invention is thus to devise a process for production of cut-off walls that represent a barrier against penetration of liquids, in which the sealing element is also plastically deformable.